This invention generally relates to a device to control torque transmitted through a vehicle""s differential and more particularly to a device that hydraulically controls torque transmission.
Differentials are typically found between drive axles in a four-wheel-drive vehicle and between the driven wheel ends in any individual drive axle. A vehicle""s inter-axle differential transmits power equally between two axles of a tandem drive axle vehicle and allows for speed differences between the two axles. A vehicle""s main differential transmits power equally between two wheel ends of an axle and compensates for speed differences between the wheels due to cornering, slightly mismatched tires, or uneven road conditions.
One disadvantage of conventional differentials is that the feature that allows for differences in axle or wheel speed also, under certain conditions, limits the amount of torque that is transmitted to the axle or wheels. Under all conditions of wheel traction there is always an equal division of axle torque between the axles or wheels. If the driving wheels encounter road surfaces of unequal traction, the wheel with the most traction will receive only as much torque as is transmitted to the wheel with the least traction. Consequently, when a wheel is spinning, the torque transmitted to it is slight and a like amount transmitted to the other wheel usually is not sufficient to propel the vehicle.
The main components of any conventional differential assembly include a differential case containing a spider, side gears, and differential pinions. Relative motion between these components allows the wheels or axles to rotate at different speeds, and this relative motion is what also limits the transfer of torque.
Mechanical devices for limiting or eliminating relative motion between differential components are well known. One common method utilized a shift collar connected through linkage to one of the differential side gears. When wheel slip occurs, the shift collar is moved to engage with a connection on the differential case. This eliminates the relative motion of components within the differential case and efficiently transfers the torque to the wheels to overcome the poor traction conditions. One disadvantage of this system is that it requires driver intervention to activate and de-activate.
Another known device used to limit slippage in differentials utilizes a hydraulic pump built into the differential to compress a clutch pack between driven wheels. When wheelspin occurs, the hydraulic pump compresses the clutch pack where friction created between plates within the clutch pack transfers drive torque to both wheels regardless of tractive conditions between the wheels and the road surface. This approach is limited primarily because of the wear that is produced within the clutch pack and the subsequent contamination of the axle lubrication fluid.
The present invention overcomes the above problems with known devices that limit slip. This invention can be use on a vehicle""s interaxle differential or main differential.
The inventive hydraulic device preferably includes a cylinder body, pistons, and a cam ring. The cylinder body is preferably generally ring-shaped with an interior bore surrounding a longitudinal axis.
Further, the cylinder body preferably has at least one pair of cylinder bores spaced radially around the cylinder body interior bore. One of the radial cylinder bores in the pair is filled with hydraulic fluid. An orifice connects the cylinder bores. The cylinder body is connected to and rotates with the differential case.
Pistons are disposed within each radial cylinder bore. Each cylinder bore pair includes a first radial cylinder bore and a second radial cylinder bore that are connected by the orifice. A first piston is disposed within the first radial cylinder bore and a second piston is disposed within the second radial cylinder bore. Since there is hydraulic fluid filling one of the radial bores in the pair, one of the pistons will always protrude from its corresponding radial bore.
The cylinder body would have as many radial cylinder bore pairs as necessary to achieve the functions described below. Further, the cylinder bores and connecting orifice are sized according to individual vehicle specifications.
The cam ring is generally circular, having an outer circumference and an inner circumference, with the inner circumference surrounding the longitudinal axis. The cam ring outer circumference is adjacent the pistons. The cam ring outer circumference also has one lobe for each radial cylinder bore pair. The lobes interact with the pistons. The cam ring is connected to and rotates ith one of the vehicle""s output shafts or side gears.
As the cylinder body and cam ring rotate relative to one another, the protruding pistons come into contact with the cam ring lobes. For each radial cylinder bore pair, the cam ring lobe pushes the protruding piston radially outward into its radial cylinder bore. This motion displaces the hydraulic fluid from one radial cylinder bore through the orifice and into the adjoining radial cylinder bore where its mating piston is forced radially inward into contact with another lobe of the cam ring. This occurs concurrently for each cylinder bore pair since the cam ring lobes and cylinder bores are all equally spaced. Resistance of the hydraulic fluid to flow from one cylinder to the other restricts the speed with which differential rotation between the differential case and the side gear can occur, causing torque transfer between the two members.
An advantage of this system is that it functions automatically without manual driver activation and contains a minimal number of additional components within the differential.
These and other features of the invention may be best understood from the following specification and drawings. The following is a brief description of the drawings.